1. Field of the Invention
The present invention relates to a braking system for use in an automotive vehicle and particularly to a hydraulic braking system comprising a master cylinder and a boost means or dynamic hydraulic braking pressure generator which outputs hydraulic power pressure supplied from a power source in response to the depression or actuation of a brake pedal.
2. Description of the Related Art
In a conventional service braking system for an automotive vehicle, there is provided a plurality of hydraulic circuits connecting wheel brake cylinders, associated with the vehicle wheels, with a hydraulic braking pressure generator such as a master cylinder so that when one of the hydraulic circuits fails braking operation is achieved by the remaining hydraulic circuits. In general, a tandem master cylinder is used in conventional dual circuit systems.
In order to reduce the force required to operate the brake pedal in the braking operation, the hydraulic braking system is provided with a servo unit which is referred as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates a hydraulic braking pressure generator, such as the master cylinder, by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal. For example, Japanese Patent Laid-Open Publication No. 59--209948 discloses a system providing hydraulic boost to a tandem master cylinder which will operate as the ordinary tandem master cylinder when the hydraulic booster is not operated.
With employment of such a hydraulic booster, it has been proposed to employ the hydraulic booster as a dynamic hydraulic pressure generator in addition to the master cylinder. In other words, a hydraulic pressure boosted by the booster (hereinafter referred to as boost pressure) in response to the depression of the brake pedal is applied directly to a hydraulic braking circuit. For example, as shown in Japanese Patent Laid-Open Publication No. 59--227552, boost pressure of the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual circuit braking system in order to reduce the stroke of the brake pedal.
Further, regarding the hydraulic braking pressure generator provided with the tandem master cylinder and the hydraulic booster, it has been proposed to connect one of the pressure chambers of the tandem master cylinder with the wheel cylinders of one of the hydraulic circuits and to output the boost pressure of the hydraulic booster to the wheel cylinders of the other hydraulic circuits via the remaining pressure chamber of the tandem master cylinder. Such a hydraulic braking system is disclosed in Japanese Patent Laid-Open Publication No. 62--155167. According to this system, various effects such as the shortening of the stroke of the brake pedal and so on are obtained.
In the above described Japanese Patent Laid-Open Publication No. 59--227552 since the boost pressure of the hydraulic booster, the dynamic hydraulic pressure, is directly supplied to the rear wheel cylinders, if the failure or loss of the hydraulic pressure occurs at the rear wheel cylinders or the hydraulic conduit connecting with the rear wheel cylinders, there is a danger of the brake fluid filling up the power pressure circuit, including the hydraulic booster. Since the hydraulic pressure of the front wheel cylinders is not generated until a piston moves a predetermined distance so as to interrupt the fluid communication between a fluid chamber and a pressure chamber, an initial idle stroke exists in the brake pedal operation so as to prevent reduction of the stroke of the brake pedal. In Japanese Patent Laid-Open Publication No. 62--155167, the boost pressure, i.e., the dynamic hydraulic pressure is directly supplied to the wheel cylinders of the other hydraulic circuits via a second fluid chamber and a second pressure chamber except for abnormal operating conditions. Accordingly, the flow of the brake fluid is assured by the communication between the wheel cylinders and the dynamic hydraulic braking pressure generator when either of the wheel cylinder of the dynamic hydraulic braking pressure generator fails. Further, a first pressure chamber is in communication with a reservoir via a compensating port until a first piston moves a predetermined distance so as to close the fluid communication between the reservoir and the first pressure chamber by a cup seal provided on the piston.